こんばんは (今晩は) is the one in the dictionary for "Good evening!". I think that こんばんわ has a cuter feeling, maybe a little softer. It's a total guess, but it might be related to the feminine わ at the end of sentences. Actually, this page seems to be saying it's related to 和 (わ - peace) which gives it a nicer feeling.

Both こんばんは and こんばんわ sound the same as the ha (は) in こんばんは sounds like wa with its current context so basically there is literally no difference between the feeling that わ adds since it doesn't make the it sound any different, just making it clear not delivering hate.